1. Field of the Invention
This invention concerns a thermionic cathode on the basis of a high-melting point support metal, an activator substance in the form of the oxide of a metal of group IIIb, a reducing agent in the form of the carbide of the support metal and an additive promoting the diffusion of the activator substance to the active surface.
2. Description of the Prior Art
Thermionic cathodes for vacuum tubes are known in the art in numerous configurations and material combinations. They range from the classic oxide cathodes with low operating temperature and limited emission-current density but long life up to complicated multi-material systems of the so-called reaction cathode type in particular. Included among the latter are, above all, cathode materials of the type W/W.sub.2 C/ThO.sub.2 ("thoriated tungsten cathodes") working via chemical exchange and supply of the activator substance from within and exhibiting a relatively high operating temperature. Such are distinguished by long life and moderate emission-current density. It has been possible to show further that the work function of the electrons could be reduced by addition of a platinum metal to the system, thus improving the latter's emission characteristics (e.g. DE-OS No. 1 614 541).
Still other material combinations are known which permit enhancement of the emission-current density with medium lifetime, among which to be chiefly mentioned are the systems Mo/Mo.sub.2 C/La.sub.2 O.sub.3 (as in DE-AS No. 2 344 936) and Mo/Mo.sub.2 C/La.sub.2 O.sub.3 /Pt-metal and the like (as in DE-AS No. 2 454 569). Thermionic cathodes are also known which are based on porous sintered masses made from powder mixtures of a high-melting point metal with a platinum metal, the pores being filled with a material containing the activator substance (e.g. DE-OS No. 2 727 187). Such cathodes are distinguished, above all, by high emission-current densities at relatively low operating temperatures.
The expected life of the aforementioned cathodes is, in view of the operating temperatures mostly still to be considered as relatively high, insufficient for many applications. It has now been found that in cathode materials containing a platinum metal as a diffusion-promoting additive (e.g. the system Mo/Mo.sub.2 C/La.sub.2 O.sub.3 Pt) both the lifetime and emissivity of the monatomic activator layer are closely dependent on the concentration and lifetime of the diffusion-promoting additive in the active surface layer of the cathode. The maintenance of a dense layer of this additive depends, on the one hand, on its evaporation rate at the surface and, on the other, on its rate of diffusion into the cathode interior. Both factors are strongly temperature dependent, so that the cathode lifetime, in the end, also has limits set from this side. In the usual reaction cathodes the operability is adversely affected from the start by the uncontrollable and inwardly diffused, into the support metal, fraction of diffusion-promoting additive so that the supply of the latter in the surface layer is prematurely exhausted.
The problem with what the invention is concerned is to develop a thermionic cathode which, independently of shape and production method as well as textural constitution, makes possible an optimal utilization of materials at high emission-current densities and guarantees the longest possible life even under the most severe operating conditions.